1. Field of the Invention
The present invention relates to a vehicle steering apparatus having a front/rear wheel (four wheel) steering function and a vehicle steering method.
2. Description of the Related Art
The four-wheel steering apparatus disclosed in Japanese Laid-Open Patent Publication No. H 8-91238 is an example of the prior art technology that facilitates maneuvering a vehicle traveling at low speed such as when the driver is attempting parallel parking. This four-wheel steering apparatus calculates a target rear wheel steering angle based upon the yaw rate, the handle angle and the like and automatically steers the rear wheels in correspondence to the calculated target rear wheel steering angle. In order to facilitate the vehicle maneuver, the rear wheels are steered freely to an intended steering angle by driving the motor regardless of the current target rear wheel steering angle by operating the right-turn switch or the left-turn switch when the vehicle is traveling at very low speed.
However, the four-wheel steering apparatus in the prior art, which requires a switch operation to be performed concurrently during a steering operation, necessitates a complicated driving operation to maneuver the vehicle traveling at low speed. The driving operation may be simplified by adopting an apparatus that steers the rear wheels by interlocking them with the front wheels. However, even when such an apparatus is employed, the vehicle maneuver is not always facilitated when the vehicle is traveling at low speed. The following is an explanation of the problems of the apparatuses described above, given in reference to simulation results.
FIGS. 12 and 13 present related art examples of the results of situations in which the driver parallel parks the vehicle while traveling forward. Rear wheels 2, which are made to interlock with front wheels 1, are steered in the reverse phase to a degree that is 0.5 times the steering quantity of the front wheels 1. The reverse phase in this context refers to turning the rear wheels to the left when the front wheels are turned to the right, for instance. FIG. 12 presents the simulation results of a steering operation performed by the driver attempting to park the vehicle parallel to a wall. As shown in the figure, the position at which the vehicle stops is set apart from the wall, and thus, the vehicle is not parked in close proximity to the wall.
FIG. 13 presents simulation results of a steering operation performed by the driver by paying closer attention to the front end and the side surface of the vehicle so that the vehicle is stopped in closer proximity to the wall. In this case, while the front end of the vehicle is positioned close to the wall, the rear end cannot easily be brought close to the wall. In other words, the four-wheel steering apparatus in the related art does not address the difficulty presented when parallel parking the vehicle in close proximity to the wall.
FIG. 14 presents a related art example of the results of a simulation of a situation in which the vehicle advances into a narrow passage. As explained above, the rear wheels 2 interlocked with the steering quantity of the front wheels 1 are steered in the reverse phase to a degree 0.5 times the steering quantity of the front wheels 1. In this case, the vehicle must be advanced forward by setting the outer front end of the vehicle further beyond the entrance to the narrow passage (above the entrance to the narrow passage in FIG. 14) and thus, there is a problem in that it is difficult for the driver to adjust the degree to which the outer end of the vehicle front is set further beyond the entrance.
An object of the present invention is to provide a vehicle steering apparatus and a vehicle steering method that facilitate parallel parking while assuring that there is a correct distance between the vehicle traveling forward and a blocking wall and allow the vehicle to drive into a narrow passage with ease without necessitating a complicated operation.
The vehicle steering apparatus according to the present invention comprises an approach disallowed area detection device that detects an approach disallowed area present toward the outside of the turning vehicle relative to the direction in which the vehicle advances, an xcex1 calculating device that calculates an angle xcex1 formed by the orientation of the boundary of the approach disallowed area detected by the approach disallowed area detection device and the orientation of the vehicle and a steering angle control device that sustains a front steering angle for wheels at the front side of the vehicle at a predetermined value equal to or lower than xcex1 and implements control on a rear steering angle for wheels at the rear side of the vehicle if the front steering angle exceeds the angle xcex1 calculated by the a calculating device.
In the vehicle steering method according to the present invention, an approach disallowed area present toward the outside of the turning vehicle relative to the direction in which the vehicle is advancing is detected, an angle xcex1 formed by the orientation of the boundary of the approach disallowed area thus detected and the orientation of the vehicle is calculated, and the front steering angle for wheels at the front side of the vehicle is sustained at a predetermined value equal to or lower than xcex1 and control is implemented on the rear steering angle for wheels at the rear side of the vehicle is advancing if the front steering angle exceeds the calculated angle xcex1.